Nasa's chief, Sean O'Keefe, has taken a step toward a robotic repair mission to save the Hubble Space Telescope.

In January, he said there would be no more space shuttle visits to service Hubble because it was too dangerous.

He has now said the US space agency would ask for proposals regarding the feasibility of a robotic servicing mission. It could take place in 2007.

His announcement was made to applause at the 204th meeting of the American Astronomical Society in Colorado.

'We must act promptly'

"In the same can-do spirit that propelled the first Hubble servicing mission, I am very pleased to inform this community that Nasa is releasing a call for proposals today for a robotic Hubble servicing capability," O'Keefe said.

The fate of Hubble has been in doubt since he said that a planned shuttle mission would not be carried out due to the safety recommendations of the Columbia Accident Investigation Board following the loss of crew and the space shuttle on 1 February 2003.

Without a service Hubble will last only a few years

The request for proposals, O'Keefe said, called for methods to de-orbit the telescope safely, to extend Hubble's service life by adding batteries and new gyroscopes, and to install new scientific instruments.

The date for submissions is 16 July. "We must act promptly to fully explore this approach," O'Keefe said.

The ideas from industry will give Nasa an indication if a robotic servicing mission can be mounted in time to rescue Hubble. Experts think it is unlikely to last beyond 2007 without attention.

'Robots can do the job'

The main aim of any robotic mission that visits Hubble will be to install some sort of de-orbiting module to allow it to be brought to Earth in a controlled crash.

Such a mission is within the capabilities developed by Nasa and other institutions, according to its chief scientist, John Grunsfeld, who flew previously on the shuttle to service Hubble.

Steven Beckwith, director of the Space Telescope Science Institute that oversees Hubble, said he was very encouraged by the news.

O'Keefe later told reporters that the Hubble telerobotic servicing was an essential technology demonstrator for the space agency's broader exploration strategy - going to the Moon and Mars.

"There may be options for extending the Hubble's useful work," O'Keefe said. "Our confidence is growing that robots can do the job."

'Unproven technology'

O'Keefe's announcement did little to dispel some people's worries about a robotic mission's ability to do the job.

Last week a petition signed by 27 astronauts was sent to President Bush.

Robotic servicing "would only be able to accomplish a portion of the tasks and would have a lower probability of success," it said.

It warned that a robotic mission "will require both the development of new and unproven technology and flying a first time ever, unmanned rendezvous and docking mission to the Hubble."

Signers included Mercury astronaut Scott Carpenter, Gene Cernan, the last man to walk on the Moon, and Robert Crippen, pilot of the first shuttle mission.